The present invention relates to a method of supplying liquid gas as fuel to an internal combustion engine. The invention furthermore relates to a fuel supply system for an internal combustion engine that is operated with liquid gas, and also to a fuel supply aggregate for such a fuel supply system.
Internal combustion engines that operate with liquid gas as fuel are finding increasing application, especially due to the improved environmental compatibility in contrast to conventional, liquid fossil fuels. In the following, the term liquid gas refers to liquefied gas, for example liquid natural gas or liquid petroleum gas, which are primarily gases based on hydrocarbons that are liquid at greater than atmospheric pressure and/or at less than the normal ambient temperature.
With the supply of such liquid gases to the mixture forming systems of internal combustion engines, which is generally effected via precisely controlled injection valves, there occurs the problem that vapor bubbles form in the liquid gas supply line that occur, for example, when, upon opening of the injection valve, the system pressure drops below the pressure at which the liquid gas is liquid at the prevailing temperature. The bubbles of vaporized liquid gas form, with the liquid phase of the liquid gas, a two-phase stream that can be accurately metered only with difficulty or barely at all, since during a prescribed injection duration the quantity of the liquid gas varies depending upon the vapor and liquid content. To resolve this problem, it is provided in the state of the art to significantly increase the system pressure beyond the two-phase equilibrium pressure, thereby preventing the formation of vapor bubbles and as a result of which the fuel remains entirely in the liquid phase. Connected with an increase in the system pressure is a significant technical expenditure for the pumps, since liquid gas has poor lubrication properties. In addition, the fuel system must have a return line, since with low fuel throughputs the pressure increase effected by the pumps is not adequate to compensate for the tendency to form vapor bubbles caused by heating.
An important characteristic of the liquid gas injection is that the quantity of fuel injected in a liquid state can during vaporization thereof rapidly absorb heat from the environment. This helps the engine for an improved fuel injection if it is running under high load since the lower intake temperatures, or the increased mixture density, achieved due to the cooling off, increase the drawn-in mixture quantity for each cycle. In addition, due to the cooling off, the proximity to the knock limit is increased. At partial load, the indicated characteristic has a drawback that due to the increased mixture density and/or the lower intake temperature a greater throttling must be effected, which leads to intake losses.
It is therefore an object of the present invention to provide a resolution for the aforementioned problems.